Musculoskeletal strain or recurrent viral infection: This is the most likely diagnosis given the history of neck pain associated with viral illnesses that resolves and then recurs with subsequent illnesses. The musculoskeletal strain could be due to coughing or other factors related to the viral infections.

Torticollis: This condition, characterized by a twisted neck, could be recurrent and might be triggered or exacerbated by viral illnesses. However, it typically presents with a noticeable head tilt, which is not mentioned.
Cervical lymphadenitis: Given the association with viral illnesses, recurrent cervical lymphadenitis could be a consideration, especially if the pain is localized to the lymph node areas.

Meningitis or subarachnoid hemorrhage: Although the child is systemically well, these conditions can present with neck pain or stiffness and are medical emergencies. The absence of fever, rash, or other systemic symptoms does not rule out these conditions entirely.
Spinal cord tumor or abscess: These conditions can cause recurrent or persistent neck pain and are critical to diagnose early to prevent long-term neurological damage.
Juvenile idiopathic arthritis: This condition can cause neck pain and stiffness, among other symptoms, and is important to diagnose to manage appropriately.

Klippel-Feil syndrome: A rare congenital condition characterized by the fusion of any two of the seven cervical vertebrae, which could potentially cause neck pain, especially if there are associated neurological or musculoskeletal issues.
Cervical spine anomalies or instability: Conditions such as atlantoaxial instability can cause recurrent neck pain, especially with certain movements or positions, and are more common in certain genetic syndromes.
Inflammatory or autoimmune conditions affecting the spine: Such as ankylosing spondylitis or other spondyloarthropathies, which are rare in young children but could present with chronic or recurrent neck pain and stiffness.